Steam reformers in the form of tubular reforming furnaces are used for producing H2/CO-containing process gases from light hydrocarbons and steam. The reformed gas, a synthesis gas rich in hydrogen, is generated by a reaction of a supplied gas (feed gas) which takes place in the catalyst tubes. Steam reformers of this type generally consist of a furnace chamber which is heated by means of a burner. The catalyst tubes are arranged in the furnace chamber and filled with a catalytic material. The catalyst tubes mostly are positioned vertically in the furnace chamber in groups spaced in parallel, wherein they are divided into several horizontal rows. The heat in the furnace chamber is transmitted to the catalyst tubes, while the same are supplied with the feed gas from above. The introduced gas is heated in the catalyst tubes, and the reaction for producing the product gas takes place. From steam and natural gas, for example H2 and CO are obtained. For discharging the product gas, an exhaust system with collecting tubes, so-called longitudinal collecting tubes, extending horizontally and vertically to the catalyst tubes is connected to the catalyst tubes, which longitudinal collecting tubes collect the product gas from the individual catalyst tubes and bundle the same in a main collecting line.
In so-called hot exhaust systems, first collecting tubes which receive the product gas of the catalyst tubes and a downstream main collecting tube which bundles the product gas from the first collecting tubes in one line are not insulated on the inside, whereas the exhaust system in its entirety is thermally insulated from outside, so that the temperature of the collecting tubes and the temperature of the outer wall of the main outlet substantially correspond to the temperature of the reformed product gas. The shell of the exhaust system thus must withstand the full heat of the product gas. Above all, it is disadvantageous that the high temperature input leads to thermal expansions in the material, which can significantly damage the structural integrity of the system. This is even amplified by recurring cooling phases. These effects are relevant above all when the vertically aligned catalyst tubes are firmly connected with the lower collecting tubes by a welded joint or the like.
The displacements and thermal expansions caused by the heat input lead to great loads and stress peaks in the connecting region between the components. In addition, there are deformations in conjunction with horizontal and vertical displacements, which additionally burden the system. To counter these effects, bent tubes known as “Pigtail” are used. The same connect the catalyst tubes with the horizontal longitudinal collecting tubes disposed thereunder and are able to compensate horizontal and vertical expansions and displacements of the longitudinal collecting tubes. It can, however, not be avoided that heating and cooling phases of the bent tubular compensators lead to stresses which are introduced into the collecting tubes. This can result in plastic deformations, whereby the longitudinal collecting tubes are shifted horizontally and are pulled or lifted in direction of the catalyst tubes. To compensate material expansions, the number of the catalyst tubes must be reduced, which limits the quantity of the catalyst tubes which can be installed inside the steam reformer furnace.
In the so-called cold exhaust systems, the interior of the exhaust system, the longitudinal collecting tubes and the main outlet line is provided with a heat-insulating layer. Due to the lining, the temperature on the outer surface of the collecting tubes is relatively low. In these systems, a compensating Pigtail connection can therefore be omitted, so that the catalyst tubes open directly into the exhaust system. For the direct connection, however, it is necessary that the longitudinal collecting tubes are provided with a number of connection openings on one side, which turns out to be complex and difficult with regard the to heat-insulating lining in the interior of the tubes, so that the insulation is not uniformly distributed. This can lead to an asymmetry with regard to the heat distribution in the upper and the lower structure of the exhaust system or the longitudinal collecting tubes, so that the same are distorted and displaced, whereby even fractures occur. Therefore, the construction of such system also involves comparatively high costs.
U.S. Pat. No. 4,647,436 A for example describes a tubular reforming furnace, in which the reaction or catalyst tubes extend out of the furnace at the bottom and outside the furnace open into a collecting line. The movements caused by the heat both in horizontal and in vertical direction of the catalyst tubes correspondingly must be taken into account in the formation of the lead-throughs through the furnace wall. To avoid damages and fractures, the same are to be dimensioned correspondingly large, in order to permit the movements of the exhaust system. Especially at these points, however, leakage can occur, so that air flows into the furnace, which effects a great heat loss inside the furnace and negatively influences the desired reactions.
Cold/hot exhaust systems represent the third group of exhaust systems and are a mixed form. In these systems, the exhaust system includes both conduits insulated on the inside and conduits not insulated on the inside. The catalyst tubes each are connected with a non-insulated collecting line, which due to the hot product gas has a correspondingly hot outer wall. The connection between collecting tube and catalyst tube is effected via Pigtail connections which have a small diameter. There are known exhaust systems in which catalyst tubes arranged in parallel are positioned grouped in rows, wherein each row is connected to a horizontal longitudinal collecting tube. The longitudinal collecting tubes are centrally connected with an internally lined coaxial main collecting tube. In general, the system is provided with two of such main collecting tubes, which are joined at one end and open into a main outlet line at one end of the reformer. Both the Pigtail connections and the hot-wall and also cold-wall collecting tubes are arranged outside the furnace.
In other cold/hot exhaust systems, the main collecting tube is arranged not coaxially, but transversely to the longitudinal collecting tubes. Beside upper ports for the catalyst tubes, the longitudinal collecting tubes have lateral ports for a transverse collector, wherein the ports for the catalyst tubes are non-uniformly distributed on both sides of the lateral port. The longitudinal collectors bundle the product gas of the catalyst tubes and are joined to one line by means of a transverse collector, wherein the transverse collector serves as main discharge line. Due to the mixed system and the great temperature differences in the materials of the tubes, differently large thermal expansions are obtained in the system, which place a mechanical load on a multitude of components of the exhaust system. The high loads in particular lead to the fact that the maximum length of the collecting tubes of the hot exhaust system and the temperature maximally tolerable by the construction are limited.
Thus, it is the object of the present invention to improve the resistance of an exhaust system for a steam reformer to thermal loads and expansions.